Stainless steel screws having selectively hardened regions are known from U.S. Pat. No. 3,376,780, issued to Tanczyn. Tanczyn discloses a stainless steel screw having selectively hardened screw flight crests and a selectively hardened head region for insertion of a screwdriver. These regions are harder than the remaining portions of the screw. The stainless steel screw has a carbon content not exceeding 0.20% by weight, a chromium content of 10-25% by weight, a nickel content of 5-20% by weight, a copper content of 1-5% by weight, and an aluminum content of 0.25-2.5% by weight. The hardening is accomplished by cold-working the stainless steel at about 700-900.degree. F., and by age-hardening at about 1050-1250.degree. F. The hardening is the greatest in the regions of the greatest cold-working.
U.S. Pat. No. 4,295,351, issued to Bjorklund et al., discloses a stainless steel screw whose flight crests have been selectively hardened. The selective hardening is achieved by aggressive cold-working of the precursor fastener blanks, at sub-zero temperatures, during formation of the threads. Another selectively hardened stainless steel screw is disclosed in U.S. Pat. No. 4,289,006 issued to Hallengren.
U.S. Pat. No. 2,229,565, issued to Hallowell Jr., discloses a socket screw whose head portion is selectively hardened. The bead portion of the screw is rapidly heated by induction to an elevated temperature. The entire screw is then quenched, causing hardening of the heated portion. The resulting screw may have a Rockwell "C" hardness ("R.sub.C ") of about 48-50 in the head region, and a lower R.sub.C of about 30-35 in the remaining portions.
U.S. Pat. No. 5,755,542, issued to Janusz et al., discloses a screw having selectively hardened threads at a lower end of the screw shank, and a selectively hardened tip. U.S. Pat. No. 5,605,423, issued to Janusz, discloses a stud having selectively hardened threads at a lower end of the stud, and a selectively hardened tip.
Certain standard carbon steel screws (having a single slot in the head) and cross-recessed screws (having two slots in the head which cross each other) can only be exposed to a limited driving torque from a driving tool (e.g. screwdriver). When the head slots are exposed to excessive turning force, the slots become enlarged and damaged, so that the driving tool can no longer effectively engage the slots.
Consideration has been given to hardening the head portion of screws to strengthen the slots. However, the hardening can cause the head and upper shank portion to become excessively brittle, resulting in 1) the head breaking from the screw shaft when excessive turning force is applied, 2) hydrogen embrittlement if the screws are plated, and 3) head-popping caused by thermal expansion and contraction of the substrate(s) to which the screw is applied, which creates stress that cannot be relieved by screw elongation. Also, selective heating of the head portion to cause hardening can result in distortion of the screw when the entire screw (having a varying temperature profile) is exposed to a quenching fluid.